1. Field of the Invention
The present invention relates to a technique of designing a semiconductor integrated circuit. In particular, the present invention relates to a technique of designing a semiconductor integrated circuit by using a TPI (Test Point Insertion) method.
This application is based upon and claims the benefit of priority from Japanese patent application No. 2007-135358, filed on May 22, 2007, the disclosure of which is incorporated herein in its entirely by reference.
2. Description of Related Art
After a semiconductor integrated circuit is manufactured, it is necessary to conduct a test for checking whether or not defects such as a delay fault (delay defect) and a stuck-at fault occur in the product. It is therefore preferable to build a test circuit that can enhance testability of the testing into the semiconductor integrated circuit in advance during a circuit design stage. Such a design technique is called “DFT (Design For Testability)”.
A “scan design” is known as one example of the DFT (refer, for example, to S. Kajihara et al., “A Framework of High-quality Transition Fault ATPG for Scan Circuits”, IEEE International Test Conference, Paper 2.1, Oct. 2006). According to the scan design, all of or a part of flip-flops included in the design circuit are replaced with scan flip-flops. At the time of the test, those scan flip-flops can constitute a certain scan path. A scan test is conducted by inputting and outputting test patterns through the scan path. The test patterns are automatically generated by an ATPG (Automatic Test Pattern Generator).
A “TPI (Test Point Insertion)” is known as a method for further facilitating the test (refer, for example, to Japanese Laid-Open Patent Applications JP-Heisei-06-331709, JP-Heisei-09-189748 and JP-2006-84427). According to the TPI, a test point is inserted at a node within the design circuit in order to improve controllability and observability of signals at the time of the test.
Recently, more attention has been focused on “small delay defect” (refer, for example, to Y. Sato et al., “Invisible Delay Quality—SDQM Model Lights Up What Could Not Be Seen”, IEEE International Test Conference, Page 47.1, Nov. 2005). With speeding up and increasing miniaturization of semiconductor manufacturing processes, even a small deviation from a design value in the critical path can cause the circuit to malfunction. That is to say, malfunction caused by the small delay defect has been increasing with speeding up and increasing miniaturization of the circuit. It is therefore important in delay testing to detect the small delay defects with high precision without overlooking them.
The inventors of the present application have recognized the following points.
FIG. 1 is a circuit diagram showing one example of a design circuit that is based on the scan design technique. The design circuit shown in FIG. 1 includes flip-flops (scan flip-flops) FF1 to FF4. A delay time of a path P1 from the flip-flop FF1 to the flip-flop FF3 is 6 ns. A delay time of a path P2 from the flip-flop FF1 to the flip-flop FF4 is 8 ns. A delay time of a path P3 from the flip-flop FF2 to the flip-flop FF3 is 3 ns. A delay time of a path P4 from the flip-flop FF2 to the flip-flop FF4 is 5 ns. A path with the maximum delay time among the paths P1 to P4 is the path P2, which is hereinafter referred to as a “longest path”. Here, let us consider a case where a small delay defect occurs at a node TN shown in FIG. 1.
FIG. 2 shows a relationship between a path used in the delay testing and a size (tdefect) of the small delay defect to be overlooked in the delay testing. In the present example, a system clock cycle is 9 ns. Since the delay time of the longest path P2 is 8 ns, a defect of not more than 1 ns does not affect a system operation. Such a defect that does not affect the system operation is called a timing redundant defect. In a case where the path P1 is used in the delay testing, a signal is transmitted within the clock cycle (9 ns) if the size (tdefect) of a delay defect is less than 3 ns. Therefore, the delay defect is not detected but overlooked in the delay testing. More specifically, a delay defect whose size (tdefect) is from 1 ns to 3 ns is overlooked when the path P1 is used. Similarly, a delay defect whose size is from 1 ns to 4 ns is overlooked when the path P4 is used. Similarly, a delay defect whose size is from 1 ns to 6 ns is overlooked when the path P3 is used. On the other hand, a delay defect is detected precisely and the overlooking does not occur in a case where the longest path P2 is used.
As described above, it is preferable to use as long path as possible in order not to overlook the small delay defect in the delay testing. In the example shown in FIG. 1 and FIG. 2, using the longest path P2 is preferable. However, which path is used in the delay testing depends on the ATPG, and there is generally a tendency that a relatively short path is used. Therefore, the small delay defect is likely to be overlooked in the delay testing. It may be possible to modify the ATPG such that the longest path is selected in the delay testing. In this case, however, the ATPG needs to generate test patterns with searching for the longest path in order to achieve the defect detection using the longest path, and thus the time for generating the test patterns becomes much longer. Moreover, constraint on signal values for testing the longest path is increased as compared with the common ATPG, which leads to increase in the number of test patterns. These cause increase in the time and cost of the delay testing.
FIG. 3 shows one example of a design circuit that is based on the typical TPI technique. As shown in FIG. 3, a test point TP (observation flip-flop) is inserted at the node TN in the design circuit shown in FIG. 1. A path from the flip-flop FF1 to the test point TP is hereinafter referred to as a “test point path PT”. The test point path PT includes the node TN, and this test point path PT is used in the delay testing.
As to the test point path PT, the setup constraint and hold constraint just need to be satisfied. Thus, the test point path PT is generally designed to be short. In many cases, the test point path PT becomes shorter than the longest path P2 according to the typical TPI technique. Therefore, the small delay defect is likely to be overlooked in the delay testing.
As described above, the small delay defect is likely to be overlooked in the delay testing, in the case of the typical TPI technique in which the path delay is not considered. The existing design technique does not support the detection of the small delay defect. If the small delay defect is overlooked during the test, a malfunction occurrence rate in the market is increased, which leads to deterioration of the product reliability.